An electric power steering apparatus which provides a steering mechanism of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies a driving force of the motor as the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus (EPS) performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of duty command values of a pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a steering wheel 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist (steering assist) command on the basis of a steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 on the basis of a current control value E obtained by performing compensation and so on with respect to the current command value. Moreover, it is also possible to receive the vehicle speed Vel from a controller area network (CAN) and so on.
The control unit 30 mainly comprises a CPU (also including an MPU), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Th detected by the torque sensor 10 and the vehicle speed Vel detected by the vehicle speed sensor 12 are inputted into a current command value calculating section 31 for calculating a current command value Iref1. The current command value calculating section 31 calculates the current command value Iref1 that is a control target value of a current supplied to the motor 20 based on the steering torque Th being inputted and the vehicle speed Vel being inputted and by means of an assist map or the like. The current command value Iref1 is inputted into a current limiting section 33 through an adding section 32A. A current command value Irefm that a maximum current is limited, is inputted into a subtracting section 32B, and a deviation I(=Irefm−Im) between the current command value Irefm and a motor current value Im being fed back, is calculated. The deviation I is inputted into a PI control section 35 for characteristic improvement of steering operations. A voltage control command value Vref that the characteristic improvement is performed by the PI control section 35, is inputted into a PWM control section 36. Furthermore, the motor 20 is PWM-driven through an inverter circuit 37 serving as a driving section. The current value Im of the motor 20 is detected by a motor current detector 38 and fed back to the subtracting section 32B. The inverter circuit 37 uses FETs as driving elements and is comprised of a bridge circuit of FETs.
Further, a compensation signal CM from a compensating section 34 is added in the adding section 32A, and compensation of the system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic and so on. The compensating section 34 adds a self-aligning torque (SAT) 343 and an inertia 342 in an adding section 344, further adds the result of addition performed in the adding section 344 and a convergence 341 in an adding section 345, and then outputs the result of addition performed in the adding section 345 as the compensation signal CM.
Such an electric power steering apparatus includes various sensors for detecting the steering angle of the column shaft (the steering shaft or the handle shaft) and for detecting a motor rotation angle and so on.
Conventionally, in order to raise a detection accuracy (detection resolution) of a rotation position by a rotation angle sensor, a rotation position detecting apparatus that combines plural sensors having displacement characteristics and detects the rotation position by a vernier (sub-scale) method, is proposed (for example, Japanese Published examined Patent Application No. H6-65967 B2: Patent Document 1). That is, the rotation position detecting apparatus of Patent Document 1 is a configuration that comprises a sensor having a first pattern of 1-pitch per rotation and a sensor having a second pattern that divides one circumference of the first pattern into “n” on the basis of a reference, and detects an absolute rotation position.